Antioxidants are widely used as nutritional supplements and believed by some to play a role in the prevention of certain diseases, such as cancer and heart disease. Antioxidant deficiency may cause oxidative stress that is harmful to cells. In contrast, prooxidants are generally believed to be harmful. The presence of prooxidants in the body has been associated with oxidative stress and linked to diseases such as hemochromatosis, Wilson's disease, and human Parkinsonism. Some prooxidants, such as adriamycin, are known to show toxicity toward cancer cells. In organisms, however, antioxidants and prooxidants are generally diffuse, and high levels can be toxic.